The present invention relates to coated fastener inserts and a method for coating such fastener inserts and, more particularly, to coating fastener inserts to prevent the galvanic corrosion in fastener assemblies.
Fastener assemblies come in a variety of shapes, sizes, designs and materials. Many fastener assemblies include not only a fastener such as a bolt, pin or screw, but also will include a fastener insert to be positioned within a tapped hole.
The type of fastener insert needed for a particular fastening operation will in large part dictated the type of fastener to be employed. While the present invention is applicable to various fastener assemblies wherein galvanic corrosion is a potential problem, the invention will hereinafter be described with reference to fastener assemblies with metallic helically coiled wire fastener inserts. By way of non-limiting example, certain metallic helically coiled wire inserts useful in association with a threaded fastener is described in U.S. Pat. No. 2,672,070 entitled Wire Coil Screw Thread Insert for Molded Material. Other fastener inserts which can be coated in accordance with the teachings of the present invention are described in U.S. Pat. Nos. 2,512,316; 2,586,007; 2,708,265; 2,755,699; 2,874,741; 2,934,123; 3,018,684, each of which is expressly incorporated by reference.
Generally speaking, tapped threads are strengthened due to the inherent flexibility of helically coiled wire inserts since the insert provides a more balanced distribution of dynamic and static loads throughout the length of thread engagement. This is especially important when the substrate including tapped holes is formed from a relatively soft metal, i.e., aluminum and magnesium. The inherent flexibility also compensates for variation in lead and angle error.
Additionally, no stress is initially introduced to the substrate because the helically coiled insert does not exhibit staking, locking or swaying and does not require keying in place. Helically coiled wire inserts allow for the use of smaller bosses, flanges and fasteners than other inserts, thus presenting a cost savings, particularly for high volume applications.
While such helically coiled wire inserts are generally useful as anchoring mechanisms for threaded fasteners in order to be used in high strength applications, such inserts must be formed from high strength metals such as 302/304 stainless steel. The use of stainless steel inserts in association with substrates or fasteners formed from other alloys leads to certain perceived problems such as the possibility of galvanic corrosion occurring over time. By the phrase "galvanic corrosion", it is meant the electrochemical corrosion resulting from the current caused in a galvanic cell between two dissimilar metals in an electrolyte because of the difference in potential (emf) of the two metals.
Stainless steel fastener inserts have been coated with zinc chromate in an effort to prevent galvanic corrosion. However, application of the zinc chromate requires strict quantitative controls and is considered labor intensive. The application of too much zinc chromate can restrict movement. Additionally, the installation tools would require frequent cleaning to prevent build up of the zinc chromate on mandrels of the tool which is undesirable. The application of too little zinc chromate leads to certain other problems such as inadequate corrosion protection, for example.
In view of the foregoing, the present invention relates to a method for preventing galvanic corrosion in fastener assemblies employing a metallic fastener insert and a fastener for use in a tapped hole, said method comprising the steps of:
a) providing a fastener and a fastener insert for retaining the fastener within said tapped hole; PA1 b) coating said fastener insert with a fluoropolymer composition; and PA1 c) adjoining the fastener and coated fastener insert within said tapped hole. PA1 a substantially cylindrical body of helically wound wire including a plurality of convolutions wherein substantially the entire exterior surface is coated with a resin bonded fluoropolymer. PA1 a) providing a metallic fastener insert; and PA1 b) coating the metallic fastener with a fluoropolymer composition to substantially encase the metallic fastener.
Optionally, the fastener insert is degreased prior to application of the fluoropolymer composition. Additionally, the fastener insert surface to be coated may be abraded using a conventional technique such as grit blasting to enhance the coating's adhesion, wear resistance and overall durability. If the coated fastener insert is likely to be used in the presence of chemicals or corrosive agents, a primer composition may be employed after degreasing but prior to coating, as will be described in greater detail below.
The present invention also relates to a coated metallic fastener insert for securing a fastener within a tapped hole, said insert comprising:
The present invention further relates to a method of producing a fastener insert which is resistant to galvanic corrosion, said method comprising the steps of:
As a result of coating the fastener insert, It is a primary object of the present invention to extend the useful life of fastener applications by preventing galvanic corrosion within a fastener assembly.